Oko satellites make up a constellation known in Russia as SPRN (a Russian abbreviation of "Satellite for Warning on Rocket Attack"). The network, which also includes dedicated ground control, is designed to detect and track launches of ballistic missiles around the world. Spacecraft in the system use sensitive telescopes to detect infrared radiation emitted by the exhaust of rocket engines. By tracking the hot plume of the rocket, the satellite can determine the trajectory of the missile.

The origin of the Soviet space-based early warning system traces its roots to 1965. At the time, the leadership of the Soviet air defenses assigned KB-1 (later OKB-41) design bureau to develop technical proposals for the possible architecture of the space-based missile warning system. Such network had already been seen as the front line in a multi-layered defense system, which could protect the USSR from a possible ballistic missile assault. From the outset, an early-warning system was tasked to detect with high precision the time and location of single or multiple missile launches 24 hours a day, 365 days a year. The project would parallel a similar development in the United States known as Defence Support Program or DSP. However unlike the American network, which would be based in the circular geostationary orbit over the Equator at the altitude of 36,000 kilometers, the Soviet early warning satellites would be sent into two types of orbits. It was a result of technological challenges facing Soviet engineers.

Known as US-K or Oko ("eye"), a Soviet spacecraft initially code-named 5B95, would be launched by Molniya rockets into highly elliptical orbits with a perigee (lowest point) at an altitude of 600 kilometers and an apogee of 40,000 kilometers above the Earth surface, while those launched by the Proton rockets would go into geostationary orbit.

The US-K constellation was developed by TsNII Kometa, while Oko satellites were built by NPO Lavochkin. A Ukrainian company Khartron based in the city of Kharkov (in the post-Soviet period known as NPP Khartron-Arkos) developed an attitude control system for 71Kh6 satellites. Soviet engineers evaluated different types of sensors which could be potentially used for the task. Eventually, ideas to use radar were abandoned in favor of television and thermal sensors.

The first-generation 5B95 satellites in the low-Earth orbit could only detect ballistic missiles against the background of space. Ultimately, both US and Soviet developers resorted to the use of infrared telescopes, which could detect the "hot" plume of a missile against the "cold" background of the Earth surface. In USSR, the telescope system for early warning satellites was identified as onboard detection hardware or BAO.

The first satellite for the US-K constellation was launched on September 19, 1972, under the name Kosmos-520. On Oct. 8, 1975, the first Soviet early warning satellite was inserted into a geostationary orbit under the official name Kosmos-775. The actual name of the payload was reportedly US-KS (where "S" stands for "stationary") or 74Kh6.

US-KMO

Around 1978, the USSR started the development of an upgraded early-warning network known as US-KMO. It included the 71Kh6 satellite for geostationary orbit and the similar 72Kh6 satellite designed to work in highly elliptical orbits. The satellites were apparently also known as Oko-1 (for geostationary orbit) and Oko-2. The first launch for the system took place on February 14, 1991, and the satellite reportedly demonstrated an excellent performance. It was followed by a second spacecraft in December 1992, also successful. Finally, a third spacecraft was launched in July 1994, officially marking the transition from the flight and design tests of the system to state validation. On Dec. 25, 1996, presidential decree No. 1770 declared the network operational. In 1998, the second phase of the US-KMO system, including Western and Eastern command posts, went through tests. The Eastern command post was first working in test mode and declared operational in 2002. (422)

Recent
launches into the SPRN constellation:

2001
Aug. 24, 23:35 Moscow Time: A Proton launched a military payload (An 71Kh6 No. 7124 (Oko-1 No. 4) early warning satellite.) The mission was delayed for 24 hours by technical
problems. The payload received the official designation Kosmos-2379.

2002 April
2: The Russian
Space Forces launched a military satellite from their Northern Cosmodrome in Plesetsk.
A four-stage Molniya-M rocket blasted off at 02:07 Moscow Time on April 2 and ten minutes later
successfully delivered a classified military payload, most likely an Oko-type
(73D6) early-warning satellite, to an initial Earth orbit. The
upper stage of the launch vehicle was then expected to maneuver the satellite
into a highly elliptical orbit. This was 220th launch for the Molniya-M
booster and was apparently delayed several times in the previous few weeks. Lubov Kudelina,
Deputy Minister of Defense and Anatoly Perminov, Chief Commander of Russian
Space Forces, KVR, personally attended the launch in Plesetsk. The payload received the official designation Kosmos-2388.

2002 Dec.
24: The Russian
Space Forces launched a military satellite from their Northern Cosmodrome in Plesetsk.
A four-stage Molniya-M rocket blasted off at 15:20 Moscow Time on December 24 and ten minutes
later successfully delivered a classified military payload, officially
identified as Kosmos-2393, to
an initial Earth orbit.
The spacecraft is most likely an Oko-type early-warning satellite. The upper stage
of the launch vehicle then maneuvered the satellite into a highly elliptical
orbit. According to official reports, the spacecraft separated from the
upper stage of the launch vehicle at 16:23 Moscow Time on December 24,
2002. The satellite established contact with the ground control center
at 17:05 Moscow Time. A major fire
at Russia's military ground control center near Moscow in 2001 is believed
to be responsible for the loss of control over one or several satellites,
providing early warning about missile attack for the Russian Ministry
of Defense.

By the end of 2013, a perigee of the inactive Kosmos-2393's orbit located over the southern hemisphere decayed to an altitude of around 90 kilometers, while an apogee was at an altitude of 16,000 kilometers. The spacecraft was first expected to reenter the atmosphere on Dec. 18, 2013, however by December 17, the reentry was predicted on December 20, according to the Interfax news agency, quoting officials at MAK Vympel corporation, a division of the Almaz-Antey consortium. According to RIA Novosti, the satellite reentered over the Pacific Ocean on Dec. 20, 2013, between 14:00 and 15:00 Moscow Time.

2003 April
24: A Proton-K rocket blasted off from
Pad 24 at Site 81 in Baikonur
Cosmodrome, Kazakhstan, at 08:23:13 Moscow Time, carrying a classified military payload, officially designated Kosmos-2397. According
to the Russian Space Forces, VKS, the vehicle successfully reached an
initial parking orbit at 8:33 Moscow Time. The Proton's upper stage then
was expected to fire twice to deliver the satellite into its final orbit.
Separation between the payload and the upper stage was scheduled for
15:00 Moscow Time on April 24. Proton
rockets are routinely used for the delivery of communications and early-warning satellites for the Russian military. The mission likely carried the Oko-1 No. 6 satellite.

2006 July 21: A four-stage Molniya rocket lifted off from Plesetsk, carrying a military satellite, officially identified as Kosmos-2422. According to official Russian sources, the payload successfully separated from the fourth stage of the launch vehicle at 09:16 Moscow Time. The mission most likely carried an Oko early-warning satellite.

2008 June 27: A Proton-K with a Block DM upper stage rocket blasted off Site 81 in Baikonur Cosmodrome, Kazakhstan, on June 27, 2008, at 03:59 Moscow Time, the official Russian ITAR-TASS news agency reported. According to the Russian Space Forces, VKS, the launch went as scheduled and at 04:06:30 Moscow Time, the vehicle established radio-contact with ground control. The separation between the payload and the upper stage was scheduled for 10:37 Moscow Time on June 27, 2008. Although the nature of the payload in this mission was not officially announced prior to launch, the Proton rockets have long been known to deliver two types of military satellites: communications and early-warning satellites. Both are delivered into geostationary orbit 36,000 kilometers above the Earth surface. Following this launch, however, the semi-official Interfax news agency quoted Aleksei Kuznetsov, secretary of Defense Minister confirming that the purpose of the launch was to replenish the SPRN early-warning system. The network is known to employ 71Kh6 satellites equipped with infra-red sensors to detect exhaust plumes of missiles.

2008 Dec. 2: The Russian military launched a military payload from the nation's northern launch site, official media reported. The Molniya-M rocket lifted off from Plesetsk Cosmodrome on Dec. 2, 2008, at 08:00 Moscow Time. The classified payload received the official name Kosmos-2446. A Western radar detected the satellite in a 518 by 39,048-kilometer orbit with an inclination 62.835 degrees toward the Equator.

2010 Sept. 30: Russia launched a classified payload into orbit, most likely an early-warning satellite to watch incoming ballistic missile launches. A Molniya-M rocket, reportedly the last in the legendary family of space vehicles, lifted off from Russia's northern launch site in Plesetsk on Sept. 30, 2010, at 21:01 Moscow Time. Lt. General Oleg Ostapenko, the commander of the Russian Space Forces, traveled to Plesetsk on the eve of the launch to personally witness the event, the official Russian media said. According to a representative of the Russian Space Forces, the liftoff went as planned and the payload was scheduled to reach its target orbit at 21:57 Moscow Time. Upon reaching the orbit, the payload received an official designation Kosmos-2469. According to an official statement, the goal of the mission was to replenish Russian military satellite constellation. The Molniya-M rocket which was used for this mission, was manufactured in 2005 and was close to the end of its 6.5-year storage warranty, the head of TsSKB Progress Aleksandr Kirilin told ITAR-TASS. With the retirement of the Molniya-M launcher, its duties to deliver similar payloads would transferred to the Soyuz-2 rocket.

2012 March 30: The eighth and last bird in the long-running family of Russian satellites designed to detect missile launches around the world blasted off from Baikonur clearing the way to a new more advanced generation of early-warning spacecraft.

The Proton-K rocket with Block-DM 2 upper stage lifted off Friday at 09:49 Moscow Time (1:49 a.m. EST) carrying a 71Kh6 satellite for the Oko-1 (eye) early-warning network of the Russian military. According to RKK Energia, which manufactured Block DM, the stage and its payload separated from the launch vehicle in the initial parking orbit. Block DM then fired for the first time at 11:03 Moscow Time. The second maneuver was initiated at 16:23 Moscow Time. Four minutes later, at 16:27 Moscow Time, the payload separated from its upper stage in a geostationary orbit at 90 degrees East longitude over the Equator. The satellite was officially designated Kosmos-2479. However insurance documents related to this mission announced the spacecraft as 71Kh6.

Before entering its final orbit, Western radar detected the satellite still connected to its upper stage in a 35,906 by 219-kilometer orbit with an inclination 49.28 degrees toward the Equator.

This mission also employed the last Proton-K rocket, marking a full transition of the Russian workhorse launcher to Proton M version.

Oko satellites would be replaced by a long-awaited EKS constellation. According to the August 2007 statement of the Commander Russia's Space Forces, KVR, Vladimir Popovkin, Russia was to start flight testing follow-on satellites for the nation's early-warning system in 2009.

As of Oct. 10, 2011, the last Oko mission was expected in December 2011 or January 2012, but by November 2011, the mission slipped to Feb. 10, 2012. By the end of 2011, the launch was expected during the fourth week of March 2012, despite alternative plans to postpone it behind another Proton mission to deliver YahSat 1B satellite.

By mid-April 2012, Kosmos-2479 drifted to a point at the geostationary orbit at 80 degrees East longitude over the Equator. However on July 19, the satellite started drifting to the East.

In June 2014, the Kommersant daily reported that Kosmos-2479 had started experiencing problems in its "foreign-supplied power batteries" as early as the beginning of that year. (The newspaper did not explain why the satellite had ended up with "foreign" power-suppy system.) In any case, despite all efforts to revive the system, the satellite reportedly stopped communicating with ground control in April and it was declared dead after only two years in operation, instead of projected five or seven years.